This application pertains to the art of steel processing. It is particularly applicable to a method and apparatus for stabilizing the oxygen content of molten steel during a casting operation. The invention is described in terms of an in the mold deoxidizing method and apparatus, although it will be appreciated that the invention has broader applications.
Heretofore, deoxidation of steel was commonly carried out in the ladle or furnace. More specifically, the steel, commonly including a deoxidizer, was brought to a molten state in the furnace and transferred to a ladle where additional deoxidizers were added. After a hold time in the ladle, the molten steel was poured from the ladle into the sprue well and through an associated runner to the molds. A common problem has been fading. That is, the amount of unreacted deoxidizer in the ladle decreases with time. Thus, the amount of unreacted deoxidizer in the ladle decreases as the molten steel is held awaiting the commencement of casting. Further, it is common to cast several molds from the runner sequentially. Additional fading tends to occur during the interval between casting the first and last mold in the sequence. This tends to cause a variation in the properties of the steel in each of the plurality of castings. To assure that an adequate amount of deoxidizer is still present in the molten steel when the last casting is made, more deoxidizer is commonly added to the ladle than is necessary for the first one or first several castings. A common deoxidizer is aluminum. It has been found, however, that when an excessive amount of aluminum is added, generally above about 0.08%, aluminum nitride embrittlement of the resultant castings tends to occur.
A runner which has some similarities to the present invention has been used in the Dunks method for changing the morphology of flake graphite in gray iron to speroidal graphite to form ductile iron. In the Dunks procedure, which is described in U.S. Pat. No. 4,004,630, issued Jan. 25, 1977 to C. M. Dunks, a nucleation agent commonly containing magnesium is added to a chamber in the runner to be dissolved into the cast iron as it moves through the runner toward the molds. The Dunks procedure, however, has a different intended purpose--the nucleation of graphite, not the reduction of oxygen. The physical parameters in iron casting are different from those in steel casting. The concentration of the nucleation agent is much higher than the concentration of deoxidizer. For example, commonly a magnesium nucleation agent is added to the cast iron which produces an alloy which contains 0.15 to 0.75% magnesium. In deoxidizing steel, aluminum deoxidizer is added in sufficiently small quantities to produce a cast steel alloy which is less than 0.08% aluminum. Magnesium is a reactive additive which boils below the temperature of the molten iron. The boiling causes a violent reaction with the iron causing agitation and infusion into the metal. Aluminum is an unreactive additive which melts but does the boil at the temperature of the molten steel. Agitation is required to dissolve and mix the aluminum through the steel. The temperatures involved in steel casting are higher than the temperatures involved in cast iron casting. Further, the addition of the deoxidizer as the molten steel moves through the runner would be expected to cause castings of poor quality. One would expect deoxidation products to impregnate the steel casting with damaging oxide inclusions.
The present invention contemplates a new and improved method and apparatus for casting steel which overcomes all of the above referenced problems and others. It provides a method and apparatus for casting steel which closely controls the amount of deoxidizer in the steel but which is readily used in conjunction with existing casting apparatus and methods.